Electron beams are used in wide variety of technological and scientific applications such as thin film deposition, electron beam welding, electron curing, waste management, ion thrusters, and plasma generators. Pulsed electron beams are often useful in such applications because they can provide improved performance compared to continuous beams. For example, in welding, instantaneous powers well in excess of average power can be achieved to increase the weld depth, while in the processing of materials, pulsed beams can drive surface chemistries while minimizing heating of the substrate, thus allowing the treatment of heat sensitive materials.
The source of electrons for an electron beam device can be a thermionic emitter (a material heated until electrons “boil off”), a field emitter (high electric fields “rip” electrons out of a material), or a plasma (ionized gas). Most devices form electron beams by biasing the source of electrons at a negative electric potential or voltage relative to earth ground; the applied electric field accelerates the electrons from the source to form an energetic beam.
Thermionic (usually a hot filament) and field emission devices are commonly used to produce electron beams in low pressure applications; at pressures exceeding 10−4 Torr these devices typically begin to operate erratically and ultimately fail, often quickly, due to ion bombardment or exposure to reactive species such as oxygen or fluorine. Furthermore, the accumulation of electron density sets the beam current in these devices. Plasma sources of electron beams, in contrast, demonstrate stable operation at high pressures, even in reactive gases, and can produce high beam currents determined mainly by the plasma density.
A plasma is an ionized gas containing equal densities of positive (ions) and negative (electrons) charges, and is typically produced by applying either a DC (continuous) or RF (oscillating) electric field to a neutral gas.
A DC plasma source requires two electrodes, a cathode and anode, with the cathode at a negative voltage relative to the anode. The discharge current runs from the anode, through the plasma, to the cathode. Near the electrodes, a thin region (sheath) of space charge exists, while the remainder of the plasma is for all practical purposes electrically neutral. These sheaths typically are ion sheaths (i.e., contain excess positive charge density), with the plasma at a higher potential than at any electrode.
A cathode that is not hot enough to emit thermionically is called a “cold” cathode. Such cold cathode discharges are the most commonly used DC plasma sources. Ion-induced secondary electron emission from the cathode is required to sustain these discharges, where the electrons liberated at the cathode surface due to ion bombardment gain enough energy as they accelerate away from the negatively biased cathode to ionize the neutral gas, thus countering the diffusive processes tending to dissipate the plasma.
When the cathode is hollow (cylindrically shaped, for example) the plasma can fill the interior of the cathode. Such hollow cathode discharges are extremely efficient plasma sources and can produce very large plasma densities. Secondary electrons are electrostatically trapped between opposing cathode walls due to the geometry of the cathode, therefore effectively guaranteeing that they will not leave the plasma volume before producing the ionization required to sustain the discharge.
A plasma can be sustained by an RF discharge in an analogous manner, except that the electrons producing the ionization gain their energy from the oscillating RF electric field rather than from the DC electric field at the cathode as in cold cathode discharges.
Independent of the method used to ionize and sustain the plasma, there will always be a flux of ions and electrons out of the plasma that is countered by the ionization source. A plasma electron beam device takes advantage of this natural flux of electrons from the plasma to produce an electron beam. Just as for thermionic or field emission sources, when the plasma is biased negatively relative to ground, electrons leaving the plasma are accelerated by the applied voltage to form the energetic electron beam.
The largest possible electron current available from a plasma source is equal to the total ion current leaving the plasma; this follows from the quasineutrality requirement, for if the two currents were not equal, ions and electrons would be leaving at different rates and the device would charge. For a two-electrode device such as a hollow cathode plasma source, this maximum electron current can be made available at the anode where it is most convenient for the purpose of producing an electron beam. This happens when the plasma potential is negative relative to the anode by several multiples of the ion temperature. In this case, ion flux to the anode is shut off since most ions will not have enough energy to climb the potential barrier, so there is only an electron flux to the anode, i.e., an electron sheath forms at the anode. For similar reasons, there is primarily ion flux to the cathode since the cathode is more negative than the plasma by several multiples of the electron temperature, i.e., an ion sheath forms at the cathode. Consequently, the only significant electron flow from the plasma occurs at the anode.
In an electron beam device using a two-electrode plasma electron source, a large fraction of the electron current to the anode becomes the beam current, limited for practical reasons by the physical transparency of the anode which is usually a wire mesh. The plasma discharge current, which is dependent most strongly on the plasma density and cathode area, with weaker dependence on the electron temperature, therefore directly determines the beam current. A negative voltage applied to the whole plasma source, typically between the anode and earth ground, determines the beam energy.
For certain combinations of beam energy, gas pressure outside the plasma source, and beam current, great care must be taken to avoid electrical breakdown in the acceleration region (i.e., between the anode of the plasma source and earth ground). The beam electrons can ionize neutral gas in this region, so maintaining separation between this beam-generated plasma and the plasma in the electron source is important. A high voltage discharge can result if the two plasmas become connected so that a conducting path to earth ground exists. This can lead to uncontrolled output, unsteady operation, component damage, or source failure. Pulsed operation of the electron beam can mitigate this problem.
Although a pulsed beam is necessary and advantageous for some applications, pulsed operation of plasma-based electron beam sources can present a number of difficulties. Often, the plasma itself is turned on and off, thus requiring the plasma to be re-established by electrical breakdown of the neutral gas (“ignition”) and then allowed to settle to the necessary operating conditions for every beam pulse. For hollow cathode plasmas, this is particularly difficult since ignition requires much higher initial neutral pressures and voltages than is needed after the plasma attains a steady state. Although ignition of the plasma is easier with an RF source, both DC and RF discharges suffer from changing plasma conditions as the plasma evolves from breakdown to a stable operating point, thus limiting the pulse duration and repetition rate.
It is therefore advantageous for the plasma source to be running continuously while the beam output is simply turned on and off. Although pulsed beam output can be obtained from a DC plasma source using a pulsed high voltage power supply to accelerate the beam, such power supplies are significantly more complicated and expensive than DC high voltage power supplies.